Changed the load specific usernames function to 'load_filter_list'.
Changed the name of the function 'write_tweets_with_links' to 'write_set_of_tuples'
Added a the function 'is_the_only_hashtag_in_text' to check if a hashtag is the only hashtag in a given text.
Added support to specify a hashtag and only the tweets with this hashtag, and only this hashtag, will be considered.
Added support to specify a hashtag and only the tweets with this hashtag, and only this hashtag, will be considered. Similarly to 'cluster_usernames'.